1. Field of the Invention
The present invention relates to a ceramic circuit board which is constructed by a ceramic substrate and metal circuit plates attached to both surfaces of the ceramic substrate.
The invention also relates to a ceramic circuit board which is constructed by a ceramic substrate, metal circuit plates attached to both surfaces of the ceramic substrate, and a metal column arranged within the ceramic substrate to connect the two metal circuit plates, and to a method for manufacturing said ceramic circuit board.
2. Description of the Related Art
In recent years, as circuit boards such as power module boards and switching module boards, ceramic circuit boards have come into common use. The examples thereof include: a ceramic circuit board constructed by bonding a metal circuit plate made of copper or the like to a metallized layer coated onto a ceramic substrate, via abrazing material such as a silver-copper alloy; a ceramic circuit board constructed by directly bonding a metal circuit plate made of copper or the like onto a ceramic substrate, via an active metal brazing material obtained by adding titanium, zirconium, hafnium, or its hydride to a silver-copper eutectic alloy; and a ceramic circuit board constructed in accordance with the so-called DBC (Direct Bond Copper) method, in which a copper plate is placed onto a ceramic substrate, and the ceramic substrate and the copper plate are directly bonded together by application of heat.
In one aspect of the prior art, in those ceramic circuit boards, in order to increase the mounting density of the metal circuit plate, the metal circuit plate is bonded both on the upper surface and on the lower surface of the ceramic substrate, and the upper and lower metal circuit plates are electrically connected to each other by a metal column arranged within a through hole formed in the ceramic substrate.
Among the aforementioned ceramic circuit boards, for example, the one constructed by directly bonding a metal circuit plate made of copper, etc. onto a ceramic substrate via an active metal brazing material is fabricated as follows. Firstly, there is prepared a ceramic substrate made of an electrically insulating material such as sintered aluminum oxide, sintered aluminum nitride, sintered silicon nitride, or sintered mullite. The ceramic substrate has a through hole drilled therethrough in the thickness direction. Secondly, a metal column is arranged within the through hole, and a brazing material paste, which is obtained by mixedly adding an organic composition or solvent to silver brazing filler powder (powder of silver-copper alloy), is applied to each end face of the metal column. Then, a metal circuit plate of predetermined pattern is placed in abutment with the ceramic substrate, with a brazing material sandwiched therebetween, such as an active metal brazing material obtained by adding at least one of titanium, zirconium, hafnium, and their hydrides to a silver-copper alloy. Lastly, the assembly thus obtained is heated in a reducing atmosphere at a temperature of approximately 900xc2x0 C. to melt the brazing material paste and the brazing material. Thereby, the metallized layer and the metal circuit plate are bonded together via the active metal brazing material, and also the metal circuit plate and the metal column are bonded together via a brazing material such as a silver brazing filler.
The ceramic circuit board thus fabricated is, after mounting thereon an electronic component such as a semiconductor device, for example an IGBT (Insulated Gate Bipolar Transistor) or an MOS-FET (Metal Oxide Semiconductor-Field Effect Transistor), using an adhesive such as solder, assembled into a resin case having an integrally-molded external input/output terminal, thereby realizing a semiconductor module. This semiconductor module finds a wider range of applications involving industrial equipment such as a robot, an actuator of an electric train, an electric vehicle, and the like, and is therefore required to operate with a high degree of reliability under harsh environments.
The conventional ceramic circuit boards, however, have the following disadvantages. If the length of the metal column is greater than the thickness of the ceramic substrate, a brazing material is absent in between the metal column and the metal circuit plate, or it is present little in quantity, if any. In this state, the metal column and the metal circuit plate are connected to each other with poor reliability, with the result that an electronic component such as a semiconductor device, which is mounted on the metal circuit plate, is unable to operate normally with stability. Moreover, if the metal column is made unduly long, it thrusts up the metal circuit plate, and consequently part of the metal circuit plate located near the metal column rises and becomes out of touch with the ceramic substrate. This causes imperfect bonding of the metal circuit plate to the ceramic substrate, resulting in the ceramic circuit board being poor in reliability.
By contrast, if the length of the metal column is smaller than the thickness of the ceramic substrate, it is difficult to supply an adequate amount of a brazing material paste, which is print-coated on the upper and lower surfaces of the metal column. If the brazing material paste is in short supply, the metal column and the metal circuit plate cannot be connected to each other by the brazing material, with the result that the ceramic circuit board is brought into an electrically opened state and thus fails to function properly. Even if the metal column can be connected to the metal circuit plate, the brazing material portion existing therebetween is so thin that it sustains high continuity resistance. Consequently, heat is generated locally at the time of energization, and this interferes with the proper and stable operation of an electronic component, such as a semiconductor devices, which is mounted on the ceramic circuit board.
In another aspect of the prior art, in those conventional ceramic circuit boards, in order to increase the mounting density of the metal circuit plate, the metal circuit plate is bonded both on the upper surface and on the lower surface of the ceramic substrate, and the upper and lower metal circuit plates are electrically connected to each other by a brazing material filled in a through hole formed in the ceramic substrate.
The aforementioned ceramic circuit boards, for example, the ceramic circuit board constructed by bonding a metal circuit plate made of copper, etc. to a metallized layer coated onto a ceramic substrate via a brazing material is fabricated as follows. Firstly, there is prepared a ceramic substrate made of an electrically insulating ceramic material such as sintered aluminum oxide, sintered aluminum nitride, sintered silicon nitride, or sintered mullite. The ceramic substrate has, on each of its upper and lower surfaces, a metallized layer of predetermined pattern, and also has a through hole drilled therethrough in the thickness direction. Secondly, the through hole of the ceramic substrate is filled with a brazing material paste obtained by mixedly adding an organic composition or solvent to silver brazing filler powder (powder of silver-copper alloy). Then, a metal circuit plate of predetermined pattern is placed in abutment with the metallized layer, with a brazing material such as a silver brazing filler sandwiched therebetween. Lastly, the assembly thus obtained is heated in a reducing atmosphere at a temperature of approximately 900xc2x0 C. to melt the brazing material paste and the brazing material. As a result, the brazing material, such as a silver brazing filler, serves to bond together the metallized layer and the metal circuit plate, and also bond together the upper and lower metal circuit plates of the ceramic substrate.
This conventional ceramic circuit board, however, poses the following problem. In the construction, the two metal circuit plates bonded to the upper and lower surfaces of the ceramic substrate are electrically connected to each other by the brazing material filled in the through hole formed in the ceramic substrate. Furthermore, the through hole formed in the ceramic substrate is filled with a brazing material paste obtained by mixedly adding an organic composition or solvent to silver brazing filler powder (powder of silver-copper alloy), and subsequently heating treatment is performed thereon at a temperature of approximately 900xc2x0 C. In this case, air existing between the individual silver brazing filler particles is contained in large quantity in the molten silver brazing filler, and the brazing material accordingly has a porous structure and its continuity resistance is as high as 7 to 10 xcexcxcexa9xc2x7cm in terms of resistivity. Thus, in the conventional ceramic circuit board, when large electric current exceeding 10A flows through the metal circuit plate and the brazing material filled in the through hole, the brazing material portion filled in the through hole undergoes resistive heat generation, and the resultant heat undesirably acts upon an electronic component, such as a semiconductor device, which is bond-fixed onto the metal circuit plate via an adhesive such as solder. As a result, the electronic component cannot be operated stably under an unduly high temperature.
The invention has been devised in view of the above-described problems with the conventional art, and one object of the invention is to provide a highly reliable ceramic circuit board in which a metal circuit plate and a ceramic substrate, as well as a metal column and the metal circuit plate, can be bonded together properly.
Another object of the invention is to provide a method for manufacturing a ceramic circuit board in which a metal circuit plate and a ceramic substrate, as well as a metal column and the metal circuit plate, can be bonded together properly with reliability, so that highly reliable electrical bonding is achieved between the metal column and the metal circuit plate.
Still another object of the invention is to provide a ceramic circuit board in which liberation of considerable amounts of heat, ascribable to resistive heat generation, can be prevented effectively, and an electronic component such as a semiconductor device, which is connected to a metal circuit plate, can be kept at a suitable temperature constantly so as to be operated normally and stably.
The invention provides a ceramic circuit board comprising:
a ceramic substrate having a through hole;
a metal column arranged within the through hole, the metal column being made 0 to 150 xcexcm shorter relative to a thickness of the ceramic substrate;
metal circuit plates attached to both surfaces of the ceramic substrate in such a way as to stop up the through hole; and
a brazing material interposed between the metal column and the metal circuit plate, for bonding together the metal column and the metal circuit plate.
According to the invention, since the metal column is made 0 to 150 xcexcm shorter relative to the thickness of the ceramic substrate, it never occurs that the metal column thrusts up the metal circuit plate, and thus the metal circuit plate can be bonded to the ceramic substrate properly. Besides, since a sufficient amount of brazing material exists in between the metal column and the metal circuit plate, the metal column and the metal circuit plate can be bonded together properly, thereby realizing a highly reliable ceramic circuit board.
The invention also provides a method for manufacturing a ceramic circuit board, comprising the steps of:
preparing a ceramic substrate having a through hole, a metal column with brazing material, and at least two pieces of metal circuit plates, said metal column with brazing material being made 40 to 140 xcexcm longer relative to a thickness of the ceramic substrate, by coating both ends of a metal column which is 0 to 150 xcexcm shorter relative to the thickness of the ceramic substrate, with a brazing material;
arranging the metal column with brazing material within the through hole of the ceramic substrate, and arranging the metal circuit plates on both surfaces of the ceramic substrate in such a way as to stop up the through hole; and
bonding, after melting the brazing material by heating, the metal column and the metal circuit plates together via the molten brazing material.
According to the invention, within the through hole of the ceramic substrate is arranged a metal column with brazing material that is made 40 to 140 xcexcm longer relative to the thickness of the ceramic substrate, by coating both ends of a metal column which is 0 to 150 xcexcm shorter relative to the thickness of the ceramic substrate, with a brazing material. With this arrangement, at the time of arranging the metal circuit plates on both surfaces of the ceramic substrate in such a way as to stop up the through hole, the metal column with brazing material is brought into contact with the metal circuit plates without fail. Moreover, since the metal column and the metal circuit plates are bonded together via an adequate amount of brazing material, the continuity resistance as observed in the junction therebetween can be kept sufficiently low, whereby making it possible to fabricate a ceramic circuit board that is free from local heat generation caused at the time of energization, and is capable of operating a component mounted thereon normally with stability. Further, since an adequate amount of brazing materials are coated at both ends of the metal column in advance, variation in the amount of the brazing material can be suppressed, resulting in an advantage in ensuring stability in the bonding status and configuration of the construction. As a result, highly reliable electrical bonding can be achieved.
In the invention, it is preferable that the metal circuit plate is made of copper or aluminum.
In the invention, it is preferable that the metal column is made of copper or aluminum.
According to the invention, since the metal circuit plate and the metal column are each made of copper or aluminum, even if large electric current flows through the metal circuit plate or the metal column, none of the metal circuit plate, the metal column, and the junction therebetween liberates considerable amounts of heat due to Joule effect. As a result, an electronic component such as a semiconductor device, bond-fixed onto the metal circuit plate using an adhesive such as solder, can be kept at a suitable temperature constantly so as to be operated normally and stably for a longer period of time.
In the invention, it is preferable that a space secured between an inner wall surface of the through hole and an outer wall surface of the metal column is kept in a range of 30 to 200 xcexcm in length.
According to the invention, the space secured between the inner wall surface of the through hole and the outer wall surface of the metal column is kept in a range of 30 to 200 xcexcm in length. With this configuration, when heat is applied to the ceramic circuit board, the inner wall surface of the through hole can be prevented from being undesirably pressed and expanded by the outer wall surface of the metal column because of the difference in thermal expansion coefficient between the ceramic substrate and the metal column. This helps prevent occurrence of crack or fracture in the ceramic substrate. Moreover, when inserted into the through hole, the metal column can be prevented from being tilted. Hence, the metal circuit plate and the metal column can be connected to each other without fail.
The invention further provides a ceramic circuit board comprising:
a ceramic substrate having a through hole;
a metal column arranged within the through hole; and
metal circuit plates attached to both surfaces of the ceramic substrate in such a way as to stop up the through hole,
wherein the metal circuit plates attached to both surfaces of the ceramic substrate are connected to each other by the metal column,
and wherein, between an inner wall surface of the through hole and an outer wall surface of the metal column is secured a space which is 30 to 200 xcexcm long.
According to the invention, a 30 to 200 xcexcm-long space is secured between the inner wall surface of the ceramic substrate""s through hole and the outer wall surface of the metal column. Thus, even if, when heat is applied to the ceramic circuit board, the outer wall surface of the metal column is expanded and swollen due to the difference in thermal expansion coefficient between the ceramic substrate and the metal column, the resultant expansion can be successfully accommodated by the space. This helps prevent the inner wall surface of the through hole from being pressed and expanded by the outer wall surface of the metal column, and thereby prevent occurrence of crack or fracture in the ceramic substrate. As a result, the ceramic circuit board can be operated normally and stably for a longer period of time.
In the invention, it is preferable that the metal circuit plate is made of copper or aluminum.
In the invention, it is preferable that the metal column is made of copper or aluminum.
According to the invention, the copper- or aluminum-made metal circuit plates, attached to both surfaces of the ceramic substrate, are electrically connected to each other by the metal column arranged within the through hole of the ceramic substrate, said metal column being made of copper or aluminum containing little pores and having a resistivity of 3 xcexcxcexa9xc2x7cm or below. By so doing, even if large electric current exceeding 10A flows through the metal circuit plate and the metal column, there occurs no liberation of considerable amounts of heat ascribable to resistive heat generation. As a result, an electronic component such as a semiconductor device, bond-fixed onto the metal circuit plate using an adhesive such as solder, can be kept at a suitable temperature constantly so as to be operated normally and stably for a longer period of time.
In the invention, it is preferable that the metal circuit plate has its surface plated with a layer made of nickel.
According to the invention, the metal circuit plate has its surface plated with a nickel-made layer which exhibits excellent conductivity, is highly corrosion-resistant, and has excellent wettability with respect to a brazing material. This makes it possible to effectively protect the metal circuit plate against oxidative corrosion, and to strengthen the electrical connection between the metal circuit plate and an external electric circuit and the connection between an electronic component such as a semiconductor device and the metal circuit plate.
In the invention, it is preferable that the plate layer is made of a nickel-phosphorus amorphous alloy containing phosphorus in an amount of 8 to 15 wt %.
According to the invention, the plate layer is made of a nickel-phosphorus amorphous alloy containing phosphorus in an amount of 8 to 15 wt %. This makes it possible to protect the surface of the plate layer against oxidation and thus maintain the wettability with respect to the solder for a longer period of time.
In the invention, it is preferable that the plate layer is 1.5 to 3 xcexcm thick.
According to the invention, since the plate layer is 1.5 to 3 xcexcm thick, the entire surface of the metal circuit plate is completely coated, whereby making it possible to effectively protect the metal circuit plate against oxidative corrosion, to suppress an undesirable increase in the internal stress developed inside the plate layer, and to prevent occurrence of warpage or crack in the ceramic substrate.
In the invention, it is preferable that the metal column has a diameter of 200 xcexcm or above.
According to the invention, since the diameter of the metal column is set at 200 xcexcm or above, even if large electric current exceeding 10A flows through the metal column, there occurs no liberation of considerable amounts of heat ascribable to resistive heat generation. As a result, an electronic component such as a semiconductor device, bond-fixed onto the metal circuit plate using an adhesive such as solder, can be kept at a suitable temperature constantly so as to be operated normally and stably for a longer period of time.